1. Field of the Invention
The present invention relates to a fuel cell stack. More particularly, the present invention relates to a fuel cell stack having end plates with a high rigidity and a stable chemical/electrochemical characteristic.
2. Description of Related Art
A proton exchange membrane fuel cell (PEMFC) is also referred to as a polymer electrolyte membrane fuel cell, and a constitution of a single fuel cell 100 is as that shown in FIG. 1, in which a central part is a membrane electrode assembly (MEA) 110, and gas diffusion layers (GDLs) 120 and 130 are disposed at two sides of the MEA 110, and are located between two bipolar plates 140 and 150. The MEA 110 is consisted of a proton exchange membrane 111 and catalyst layers 112 and 113 coated at two sides of the proton exchange membrane 111. After reaction fluid required by the fuel cell 100 is distributed by flow channels 160 and 170 in the bipolar plates and the GDLs 120 and 130, an electrochemical reaction is occurred at the catalyst layers 112 and 113. The reaction fluid required by an anode side of the fuel cell 100 is hydrogen or humid hydrogen, and when the reaction fluid contacts the catalyst layer 112 of the MEA 110 at the anode side, an oxidation reaction is occurred: H2→2H++2e−. Electrons generated by the oxidation reaction are conducted by an external circuit, and hydrogen ions can pass through the proton exchange membrane 111 and get to a cathode side of the MEA 110, so that with assistance of humid oxygen or humid air at the cathode side, a reduction reaction: O2+4H++4e−→2H2O is occurred on the catalyst layer 113 of the MEA 110 at the cathode side. It should be noticed that the proton exchange membrane 111 is a membrane containing water, so that only the hydrogen ions can pass though the water molecules contained in the proton exchange membrane 111, and other gas molecules cannot pass there through.
According to the above description, it is known that the fuel cell 100 generates power through the electrochemical reaction between the hydrogen and the oxygen, and a reaction outcome is clean water, which will not cause pollution to the environment. Since the fuel cell has advantages of high efficiency and fast response, etc, it is regarded as one of the alternative energy sources of the future. Moreover, the single fuel cell 100 can be stacked in serial to form a fuel cell stack as that shown in FIG. 2, so as to increase an output voltage to meet different power demands and applications. FIG. 2 is a side view of a conventional fuel cell stack, in which two end plates 210 and 220 located at two sides and a plurality of fastening elements 230 are used to tightly stack a plurality of single fuel cells 100, reaction fluid 261 enters the fuel cell stack 200 through a reaction fluid inlet manifold 260 and is uniformly distributed to each of the single fuel cell 100. The electrons generated by the electrochemical reaction are conducted to external for utilization through current collectors 240 and 250 located at two sides of the fuel cell stack 200, and reacted fluid 271 flows outside the fuel cell stack 200 through a reaction fluid outlet manifold 270. Moreover, cooling fluid 282 enters the fuel cell stack 200 through a cooling fluid inlet manifold 280 to maintain a suitable temperature of the fuel cell stack 200 during operation, and cooled fluid 283 can be smoothly exhausted from the fuel cell stack 200 through a cooling fluid outlet manifold 281.
One of key factors that influences a performance of the fuel cell stack 200 is a clamping pressure provided by the two end plates 210 and 220 and the fastening elements 230 when the fuel cell stack 200 is assembled. Referring to FIG. 1 and FIG. 2, when the clamping pressure is too great, the MEA 110 is deformed or even damaged due to the pressure, which may cause a decline of a transmission capacity of the hydrogen ions. When the clamping pressure is inadequate, an interface contact resistance between the MEA 110 and the bipolar plates 140 and 150 is increased, which may also cause a decline of the performance of the fuel cell stack 200. Another factor that influences the performance of the fuel cell stack 200 is stability of chemical/electrochemical characteristics of a material of the end plates 210 and 220. The reaction fluid outlet/inlet manifolds 270 and 260 and the cooling fluid outlet/inlet manifolds 281 and 280 of the end plates 210 and 220 are mainly used for guiding the reaction fluid 261 and 271 and the cooling fluid 282 and 283 with a temperature of 60-80° C. and a relative humidity of more than 90%, Unstable chemical/electrochemical characteristics of the material of the end plates 210 and 220 may lead to corrosion and exfoliation of the manifold surface, and exfoliations can block the flow channels 160 and 170, and accordingly the MEA 110 is contaminated and the performance of the fuel cell stack 200 is decreased.
In summary, the end plates 210 and 220 and the fastening elements 230 are not only required to provide a uniform clamping pressure when the single fuel cells are assembled, but also the end plates 210 and 220 are required to have a high rigidity and a stable chemical/electrochemical characteristic under the operation temperature, humidity and pressure of the fuel cell stack 200, so as to maintain the performance of the fuel cell stack 200 and prolong a lifespan of the fuel cell stack 200.